We are going live with 6.2 in April and have been testing for almost a year now. The first thing we did was get it installed on a new server, get our tiers in place, and then did a test conversion to see what happened. From there, you'll have to test basically everything you use in Vision to make sure it works the same. We also have a lot of custom reports, invoice templates and interfaces but did not start to tackle those until we had our new environment in place and stable (and had done multiple database conversions). Once the custom work was done, we did load testing with as many simultaneous users as we could get to participate which uncovered new issues related to workstations that we are resolving now. We're planning additional load testing in the next two weeks to see what else we find. I handle the front end of the application, but I will share what my DBA had to say - basically that Deltek's documentation on the technical side leaves a lot to be desired.
Bottom line, test it, test it, and then test it again!